Level control for rotating furnaces

ABSTRACT

A horizontally mounted rotary cylindrical furnace is provided with an internal helical flight for at least the preponderant part of one revolution but not more than two revolutions of the furnace so that two zones are formed within the rotary furnace. The pitch of the flight can be controlled so that material fed to the first zone is conveyed to the second zone at a rate greater than the rate of feed to the first zone. The height of the helical flight can be set so a deeper bed and greater retention time is obtained in the solids down stream of the flight. Also disclosed is a discharge mechanism which is a tapered helical rotor with its larger vanes being coextensive with and fixed to the furnace diameter and the smaller vanes being mounted in an co-extensive with and fixed to a take-off conveyer tube.

United States Patent 1 Bell 11 3,770,257 51 Nov. 6, 1973 I LEVEL CONTROLFOR ROTATING FURNACES [75] Inventor: James Alexander Evert Bell,

Port Colborne, Ontario, Canada [73] Assignee: The International NickelCompany, Inc., New York, N.Y.

[22] Filed: July 2, 1971 21 Appl. No.: 159,259

[30] Foreign Application Priority Data July 8, 1970 Canada 087,693

[52] US. Cl. 266/18, 75/33 [51] Int. Cl. F27b 7/16 [58] Field of Search266/36 H, 39, 18,

[56] References Cited UNITED STATES PATENTS Primary Examiner-Gerald A.Dost Att0rneyMaurice L. Pine] 57 ABSTRACT A horizontally mounted rotarycylindrical furnace is provided with an internal helical flight for atleast the preponderant part of one revolution but not more than tworevolutions of the furnace so that two zones are formed within therotary furnace. The pitch of the flight can be controlled so thatmaterial fed to the first zone is conveyed to the second zone at a rategreater than the rateof feedto the first zone. The height of the helicalflight can be set so a deeper bed and greater retention time is obtainedin the solids down stream of the flight. Also disclosed is a dischargemechanism which is a tapered helical rotor with its larger vanes beingcoextensive with and fixed to the furnace diameter and the smaller vanesbeing mounted in an coextensive with and fixed to a take-off conveyertube.

7 Claims, 3 Drawing Figures LEVEL CONTROL FOR ROTATING FURNACES Thepresent invention pertains to furnaces, and more particularly to rotaryfurnaces and processes conducted therein.

The present invention is particularly applicable to rotary furnaceswhich are employed in the reduction, oxidation or other heat treatmentof ores, ore concentrates, or other metallurgical intermediates and willbe described with particular reference thereto although it will beappreciated .that the invention has broader applications such as in theuse in cement kilns or in any other process apparatus in which acontinuous process is dependent on distinct chemical or physicalmechanisms that involve different treating times.

Rotary metallurgical furnaces have heretofore comprised a horizontallymounted and refractory-lined cylindrical steel shell, heat generatingmeans to heat and to maintain the material being treated at treatingtemperatures, atmosphere control means and means for rotating thefurnace. In order to transport material from one end of the furnace tothe other, rotary furnaces have most often been mounted at a slightinclination from the horizontal and in some instances, flights, bothlateral and helical, and dams have been employed to control the rate offlow of material through the entire furnace.

Rotary furnaces have also been provided with means for feeding materialto one end and with means for withdrawing the material from the otherend. Charging raw material to rotary furnaces has not presented manyproblems but difficulties are often encountered in discharging processedmaterial therefrom. Occasionally, the desired metallurgical treatment isconducted at other than atmospheric pressures and such pressures,whether subatmospheric or superatmospheric, must be maintainedthroughout the furnace during discharge which, if continuous, presentsnumerous problems.

Rotary furnaces, as described hereinbefore, have been used, and arecurrently being used, to reduce ores, ore concentrates and othermetallurgical intermediates. For example, iron oxides containing nickelare selectively reduced to reduce substantially all the nickel values tometallic nickel while reducing only controlled amounts of iron. Theselectively reduced ore is then treated, either hydrometallurgically orvapometallurgically, to recover the reduced nickel values.

v the furnace if the discharging end is provided with a As with manychemical reactions, the chemical reduction of metallurgical oresinvolves two distinct processes. The ore must first be preheated to thereduction temperature, and thereafter the heated ore is reduced with anappropriate reductant. Preheating involves well understood heat exchangeprinciples and is a relatively fast process since the ore, which is notgenerally massive, presents large surfacevar eas at and through whichheat exchange rapidly and readily occurs. On the other hand, chemicalreduction, when effected in the solid state, is a relatively slowprocess since diffusional processes are involved. Advantageously, theore is preheated as fast as possible while ja longer period is allowedfor chemical reduction, but this ideal has rarely been realized inactual practice.

7 In order to realize more efficient chemical reduction of metallurgicalores in rotary furnaces, the bed of ore in a preheating zone should berelatively shallow in order to increase the rate of heat exchange whilethe bed of ore in a reducing zone is maintained relatively dam. However,the increase in depth of the bed is not so great as to significantlyimprove the efficiency of the preheating operation or to materiallyincrease the time of residency'in the reducing zone. Also, it ispossible, as has been suggested, to decrease the depth of the bed in apreheating zone-by employing flights, both' lateral and'transve'rsethroughout the entire preheating zone. Although flights in a preheatingzone can increase the preheating'rate, problems associated with dustingare encountered and the flights, extending throughout the preheatingzone, provide extensive surfaces and angles over which and inwhichmassive accretions can rapidly form. At times, such massive accretionsbuild up so rapidly that any advantage gained by improved preheating islost by the time required to remove such accretions.

As noted hereinbefore, metallurgical rotary furnaces can be operated atsubatmospheric or superatmospheric pressures for thermodynamic andkinetic reasons. When furnaces are operated under subatmospheric orsuperatmospheric pressures and on a continuous basis, as rotary furnacesare, discharging treated materials can present numerous problems. Evenwhen operated at ambient pressures discharging treated material fromrotary furnaces can be troublesome if it is required to maintain aneutral or protective atmosphere over the treated material while leavingthe atmosphere in the furnaceundisturbed. Frequently, the problemsassociated with discharging treated material were overcome by enclosingthe entire discharge end of the furnace with a separate and distincthousing which acted as a pressure seal. The discharge ends of rotarykilns have also been provided with pressure seals, but the manufactureand maintenance of gas tight sealsof the size required to sealcommercial rotary furnaces whereboth difficult and expensive. Althoughmany attempts were made to overcome the foregoing difficulties and otherdisadvantages, none, as far as I am aware, was entirely successful whencarried into practice commercially on an industrial scale.

It has now been discovered that horizontally mounted rotatingcylindrical furnaces can be improved by providing the furnace with ahelical flight to establish two zones along the longitudinal axis of thefurnace to control the rate of flow of material from one zone to theother. The diameter of the helical flight can be arranged so that thebed depth down stream from the flight is increased-Such rotary furnacescan be further improved by providing the discharge end of the furnacewith a discharge tube and a helical'rotor with the larger vanesof therotor being coextensive with and fixed to the inside diameter of thefurnace and the smaller vanes being coextensive with and fixed to theinternal diameter of the discharge'tube.

In accordance with the present invention, a rotary furnace of the typedescribed is provided wherein the furnace has an internal helical flightfor at least a pre ponderant part of one revolution but not more thanabout two revolutions of the furnace so that the furnace is separatedinto two zones along its longitudinal axis and so that the rate ofpassage of material through the two zones can be independently varied.In this fashion the depth of material in each section and the retentiontime in each section can be varied. I

It is an object of the present invention to provide a rotary furnacewhich has at least two zones and in which material passes through thezones at different rates. 1

Another object of the present invention is to provide a cylindricalfurnace in which the rate of flow of material through various zonestherein can be controlled and which has an improved discharge mechanism.

Other objects and advantages will become apparent from the'followingdescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a longitudinal cross-sectional view of a rotary furnace inaccordance with the present invention;

FIG. 2 is a cross-sectional view of the furnace taken along the lines 22in FIG. 1; and

FIG. 3 is a cross-sectional view showing an improved discharge mechanismof a rotary furnace in accordance with the present invention.

Referring now to the drawings wherein the showings are for the purposeof illustrating the preferred embodiments of the invention only and notfor the purpose of limiting same, FIG. 1 shows an improved rotaryfurnace in accordance with the present invention. The rotary furnace Aincludes a cylindrical shell 12, generally steel, which is suitablylined with refractory 13. The charging end of the furnace A is generallyrepresented at B and includes a flue 14 and a feeding device 15. Thedischarging end of the furnace A, generally depicted at C, is providedwith burner 16 and a housing 17, which housing acts as a gas seal and adischarge hopper. The cylindrical shell is provided with steel tires 18which in conjunction with driving means 19 rotate the cylindricalfurnace at preselected speeds. Optionally, the rotary furnace can beprovided with air vents or side burners 20. In accordance with thepresent invention, the cylindrical furnace is provided with a helicalflight 21 which can be constructed of the same material as that of therefractory lining 13, or a suitable metallic material. Since, asdescribed hereinafter, helical flight 21 has a pitch such thatparticulate material is conveyed from one zone to another at a rategreater than that provided by gravity and the inclination and rotationof the furnace, dam 22 can optionally be located immediately beforeflight 21 to provide more readily controllable depths in the first zone.

The helical flight is an important feature of the present invention. Theflight can be of any height as long as it is at least as high as thedepth of the deepest bed. For example, as shown in FIG. 1, if thefurnace is a metallurgical furnace in which an oxide is preheated inzone D and reduced in zone B, the ore in zone E is maintained at agreater depth than in the preheating zone D to provide a longerresidence time in zone E and the height of the flight is at least equalto the depth of the ore in zone E. Placement of the helical flight alongthe longitudinal axis of the cylindrical furnace is not mechanicallycritical. However, such' placement can be of great importance forparticular chemical or metallurgical operations. If the furnace isemployed to directly reduce metal oxides, as iron oxide or nickel oxide,the helical flight is placed at the position in the furnace where ashallow bed and the metal oxide will have been preheated to effect thereducing temperatures.

When reducing nickeliferous iron oxide ores with liquid hydrocarbons, asdescribed in Canadian Pat. No. 744.329 the helical flight is placed atthat position where a shallow bed of the ore has been preheated to atemperature of at least about 900C.

The pitch of the helical flight can be varied to suit individualrequirements. In metallurgical application, the inclination of thefurnace will cause material to flow through the furnace, and the helicalflight will have a pitch such that the flow of material will be greaterthan that caused by the inclination of the furnace, i.e., the leadingedge of the helical flight cuts into the ore flowing along the furnacelongitudinally under the impetus of gravity and the usual slightinclination of the furnace toward the discharge end. For example, inreducing iron oxide, the pitch of the helical flight can be such thatpreheated ore in zone D is transferred to the reducing zone E at a rategreater than the rate of feeding iron oxide to the preheating zone Dwhereby the preheating operation is made more efficient and theresidence time of iron oxide in the reducing zone is increased. It willbe noted that in metallurgical furnaces drying or preheating will beconducted in the first zone or zones and chemical processes are effectedthereafter. Since these chemical reactions will most frequently bediffusion controlled processes, it is desirable to have the bed ofmaterial shallow in the initial zones and deeper thereafter.

In addition to the foregoing embodiments, it might be noted that morethan one helical flight can be employed in a single cylindrical furnaceso that three or more distinct zones can be established. By regulatingthe pitch of each of the flights, zones of different depths can beformed so that surface area dependent operations can be conducted moreefficiently in shallow beds while diffusion controlled (time dependent)operations are affected with deeper beds.

FIG. 3 depicts a preferred embodiment of the present invention. Only thedischarge end of the cylindrical furnace is shown in FIG. 3 since thepreferred embodiment relates to a discharging mechanism. The cylindricalfurnace includes a cylindrical steel shell 30 which is lined with asuitable refractory 31 the furnace is rotated about itslongitudinal-axis by drive means (not shown in the drawing). Flow ofmaterial through the furnace is controlled by helical flights asdescribed hereinbefore. The furnace is provided with a fixed dischargetube 32 which can be a steel shell 33, suitably lined with refractory 34to handle hot material. The discharge tube is provided with a gas seal35 so that the escape of furnace gases through the discharge end areminimized. It might be noted that an advantageous feature of the presentinvention is that the diameter of the discharge tube is vconsiderablysmaller than that of the cylindrical furnace so that any gas seals thatare provided are more effective since it is well known that smallerfixtures are easier to seal than are larger ones. The dischargemechanism is a conveyor screw 36 with helical flights 38 extending intothe furnace being substantially equal to the internal diameter of thefurnace while those helical flights 39 extending to the discharge tubeare substantially coextensive with the internal diameter of thedischarge tube. The helical flights for the discharge mechanism can bemade of suitable metals or refractories. The screw 36 is fixedly mountedin the furnace so that it rotates with the furnace. Thus, flights 39rotate in fixed discharge tube 32 as the furnace rotates.

The pitch of flights 39 is regulated so that the material is fed outfaster than it arrives. The rate of discharge can be effectivelycontrolled by regulating the rotation of the conveyor screw.

The shaft upon which the helical screw is mounted can be hollow so thatgases such as air and/or fuel can be introduced into the interior of thefurnace so that the shaft can act as a burner for supplying heat to thecylindrical rotating furnace. Thus, acting as a burner the rate of airand fuel can be varied to provide oxidizing and reducing conditions atthat portion of the rotating furnace into which the shaft extends.

It will be appreciated by those skilled in the art that rotary furnacesequipped with a helical flight in accordance with the apparatus of thepresent invention pro- I vide optimum conditions for processingparticulate material where different operations are dependent on variousphysical processes. Thus, in a preheating zone where heat exchange ismore dependent on surface area a shallow bed is provided to promoteheating while in a reducing zone where chemical diffusion, dependent ontime and temperature, is the major consideration a longer residence timeis provided while heat losses are minimized.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. In a rotary furnace including a cylindrical furnace having a chargingend, a discharging end and a substantially uniform cross section andbeing rotatable about its longitudinal axis and mounted at a preselectedinclination so that rotation of the furnace transports particulatematerial fed to the charging end to the discharging end, the improvementwhich comprises: at least one helical flight mounted on the furnacewalls between said charging and discharging ends forming within saidfurnace two zones, said helical flight extending for at least thepreponderant part of one revolution of said furnace but not more thantwo revolutions of said furnace, so that the rate of flow of particulatematerial through the zones can be controlled by controlling the speed ofrotation of the rotary furnace and a conveyer tube fixedly mounted atthe discharging end of the furnace, a conveyor screw having helicalflights that are substantially coextensive with the internal diameter ofthe conveyor tube and the furnace-and fixed to the furnace wall so thatparticulate material is conveyed out of the furnace into the conveyortube by the helical flights by rotation of the furnace.

2. In a rotary furnace including a cylindrical furnace having a chargingend, a discharging end and a substantially uniform cross section andbeing rotatable about its longitudinal axis and mounted at a preselectedinclination so that rotation of the furnace transports particulatematerial fed to the charging end to the discharging end, theimprovementwhich conprises: an internal helical flight for apreponderant part of at least one revolution of the furnace mounted on afurnace wall between the ends of the furnace to provide the furnace withtwo separate zones along its longitudinal axis whereby the depth ofparticulate material in the respective zones can be controlled byregulating the speed of revolution of the furnace and a conveyor tubefixedly mounted at the discharging end of the furnace, a conveyer screwhavinghelical flights that are substantially coextensive with theinternal diameter of the conveyor tube and the furnace and fixed to thefurnace wall so that particulate material is conveyed out of the furnaceinto the conveyor tube by the helical flights by rotation of thefurnace.

3. In a rotary furnace including a cylindrical furnace having a chargingend, a discharging end and a substantially uniform cross section andbeing rotatable about its longitudinal axis and mounted at a preselectedincliulate material fed to the charging end to the discharging end, theimprovement which comprises: an internal helical flight for thepreponderant part of at least one revolution but not more than tworevolutions of the furnace mounted on the furnace wall and between theends of the furnace to provide two zones along the longitudinal axis ofthe furnace whereby particulate material treated in the furnace can becontrolled to have different depths within the two zones in the furnaceand a conveyer tube fixedly mounted at the discharging end of thefurnace, a conveyer screw having helical flights that are substantiallycoextensive with the internal diameter of the conveyor tube and thefurnace and fixed to the furnace wall so that particulate material isconveyed out of the furnace into the conveyor tube by the helicalflights by rotation of the furnace.

4. The furnace as described in claim 3 wherein the furnace is mounted ata preselected inclination from the horizontal to cause particulatematerial to be transported through the furnace and the helical flighthas a pitch such that the particulate material flow through the flightis greater than that caused by the preselected inclination so thatparticulate material in the zone nearer the discharging end is deeperthan particulate material in the zone nearer the charging end.

5. The furnace as described in claim 4 further including a damproximately mounted to the helical flight so that the dam controls thedepth of particulate material nearer the charging end and the helicalflight feeds par-- ticulate material to the zone nearer the dischargingend.

6. The furnace as described in claim 5 wherein the helical flight has aheight at least as high as the depth of the deepest bed.

a hollow shaft onwhich the conveyor screw is mounted so that the hollowshaft can function as a burner.

1. In a rotary furnace including a cylindrical furnace having a chargingend, a discharging end and a substantially uniform cross section andbeing rotatable about its longitudinal axis and mounted at a preselectedinclination so that rotation of the furnace transports particulatematerial fed to the charging end to the discharging end, the improvementwhich comprises: at least one helical flight mounted on the furnacewalls between said charging and discharging ends forming within saidfurnace two zones, said helical flight extending for at least thepreponderant part of one revolution of said furnace but not more thantwo revolutions of said furnace, so that the rate of flow of particulatematerial through the zones can be controlled by controlling the speed ofrotation of the rotary furnace and a conveyer tube fixedly mounted atthe discharging end of the furnace, a conveyor screw having helicalflights that are substantially coextensive with the internal diameter ofthe conveyor tube and the furnace and fixed to the furnace wall so thatparticulate material is conveyed out of the furnace into the conveyortube by the helical flights by rotation of the furnace.
 2. In a rotaryfurnace including a cylindrical furnace having a charging end, adischarging end and a substantially uniform cross section and beingrotatable about its longitudinal axis and mounted at a preselectedinclination so that rotation of the furnace transports particulatematerial fed to the charging end to the discharging end, the improvementwhich conprises: an internal helical flight for a preponderant part ofat least one revolution of the furnace mounted on a furnace wall betweenthe ends of the furnace to provide the furnace with two separate zonesalong its longitudinal axis whereby the depth of particulate material inthe respective zones can be controlled by regulating the speed ofrevolution of the furnace and a conveyor tube fixedly mounted at thedischarging end of the furnace, a conveyer screw having helical flightsthat are substantially coextensive with the internal diameter of theconveyor tube and the furnace and fixed to the furnace wall so thatparticulate material is conveyed out of the furnace into the conveyortube by the helical flights by rotation of the furnace.
 3. In a rotaryfurnace including a cylindrical furnace having a charging end, adischarging end and a substantially uniform cross section and beingrotatable about its longitudinal axis and mounted at a preselectedinclination so that rotation of the furnace transports particulatematerial fed to the charging end to the discharging end, the improvementwhich comprises: an internal helical flight for the preponderant part ofat least one revolution but not more than two revolutions of the furnacemounted on the furnace wall and between the ends of the furnace toprovide tWo zones along the longitudinal axis of the furnace wherebyparticulate material treated in the furnace can be controlled to havedifferent depths within the two zones in the furnace and a conveyer tubefixedly mounted at the discharging end of the furnace, a conveyer screwhaving helical flights that are substantially coextensive with theinternal diameter of the conveyor tube and the furnace and fixed to thefurnace wall so that particulate material is conveyed out of the furnaceinto the conveyor tube by the helical flights by rotation of thefurnace.
 4. The furnace as described in claim 3 wherein the furnace ismounted at a preselected inclination from the horizontal to causeparticulate material to be transported through the furnace and thehelical flight has a pitch such that the particulate material flowthrough the flight is greater than that caused by the preselectedinclination so that particulate material in the zone nearer thedischarging end is deeper than particulate material in the zone nearerthe charging end.
 5. The furnace as described in claim 4 furtherincluding a dam proximately mounted to the helical flight so that thedam controls the depth of particulate material nearer the charging endand the helical flight feeds particulate material to the zone nearer thedischarging end.
 6. The furnace as described in claim 5 wherein thehelical flight has a height at least as high as the depth of the deepestbed.
 7. The furnace described in claim 3 further including a hollowshaft on which the conveyor screw is mounted so that the hollow shaftcan function as a burner.